This invention relates to a printed wiring board having a structure for improving electromagnetic environmental compatibility (hereinafter abbreviated to EMC).
With a recent development of a digital technique, degree of integration of LSI (Large Scale Integrated Circuit) and density of a printed wiring board have been increased in recent years, and operating speed thereof has also been increased.
Such a high-speed digital circuit includes a digital signal containing many high-frequency components. Consequently, it is necessary to suppress radioactive noise due to unnecessary radiation for a printed wiring board having a digital circuit.
Furthermore, since resistance of the printed wiring board against external noise has become lower due to the increase in the density and operating speed of digital devices, it is necessary to take measures for improving noise resistance of a printed wiring board.
In order to improve EMC of such digital devices and circuit, a common mode choke conventionally has been mounted onto the printed wiring board.
In FIG. 1, a reference numeral 11 denotes a LSI. The LSI has circuit blocks CB1-CBn which are connected to pins 13a1, b1 to 13an, bn for communication lines and a common E (earth or ground) pin 12.
The pins 13a1, b1 to 13an, bn for the communication lines are connected to common mode chokes 15-1 to 15-n for preventing common mode noise via the communication lines 14a1, b1 to 14an, bn. 
Capacitors 16a1, b1 to 16an, bn having high-frequency characteristics are connected to portions of the communication lines between the common mode chokes 15-1 to 15-n and terminals 17a1, b1 to 17an, bn. On the other hand, the other terminals of the capacitors 16a1, b1 to 16an, bnare connected to a solid E (earth) layer 32 illustrated in FIG. 2.
Although a pair of communication lines 14a1, 14b1 alone are shown in FIG. 2, similar common mode noise may occur in the other communication lines. Reference numeral Nab in the same drawing denotes common mode noise occurring in the E pin 12. When the common mode noise Nab generates in the E pin 12, common mode noise Na appears for the communication line 14a1, while common mode noise Nb generates for the communication line 14b1.
Under this circumstance, a common mode current Ia passing through the communication line 14a1 flows along a path which takes a round via the common mode choke 15-1, capacitor 16a1, solid E layer 32 and communication pin 13a1. Thereby, the common mode current Ia generates magnetic flux xcfx86a.
In the meantime, a common mode current Ib passing through the communication line 14b1 flows along a path which takes a round via the common mode choke 15-1, capacitor 16b1, solid E layer 32 and communication pin 13b1. Thereby, the common mode current Ib generates magnetic flux xcfx86b.
Since the units of magnetic flux a, b generate in the reverse directions to each other, a, b offset each other. In consequence, the common mode noises Na, Nb are attenuated with the result that levels of the common mode noises at the terminals 17a1, 17b1 are lowered.
As described above, expensive common mode chokes must be used to suppress the occurrence of common mode noise of the digital circuit and to improve EMC in the related printed wiring board.
It is therefore an object of this invention to provide a printed wiring board which is capable of suppressing common mode noise without using expensive parts, such as, common mode chokes as EMC improving devices.
According to an aspect of the present invention, the printed wiring board capable of solving the above-mentioned problems has a laminate formed of a pattern-wired layer on which plural communication lines are print-wired, an insulating layer, and a grounding layer, the printed wiring board including slotted portions (groove portions) provided in the sections of the grounding layer which are positioned under two parallel-printwired communication lines, and capacitors provided between the slotted portions and external connecting terminals of the printed wiring board, and connected at one terminal thereof to the communication lines, and at the other terminal thereof to the grounding layer through a through hole provided in the insulating layer.
According to another aspect of the present invention, the width of each of the slotted portions of the first-described printed wiring board is determined on the basis of a distance between the two parallel-print-wired communication lines.
According to still another aspect of the present invention, each of the capacitors in the first-described printed wiring board may be formed of the pattern-wired layer, an insulator provided on the insulating layer and having a dielectric constant different from that of the insulating layer, and the grounding layer.